The present invention relates to a sheet feeder for an electrophotographic copier or like image forming apparatus and, more particularly, to a sheet feeder with a structure for aligning the trailing edges of a large amount of sheets which are stacked on a tray.
In an image forming apparatus such as an electrophotographic copier, it is a common practice to feed a plain paper sheet or similar sheet to a photoconductive element which carries a developed image thereon or to a transfer element to which an image is to be transferred from the photoconductive element, thereby producing a copy. A sheet feeder is associated with an image forming apparatus for implementing such sheet feed. To cope with the current tendency toward a greater number of copies, a sheet feeder capable of accommodating a large amount of paper sheets at a time has recently come into the market.
A prior art sheet feeder is located in the vicinity of a sheet feed section of an image forming apparatus. The sheet feeder is generally made up of an elevatable tray which is loaded with a stack of paper sheets, and a separating mechanism for feeding the sheets one by one from the tray toward the sheet feed section of the image forming apparatus. The separating mechanism is constituted by a draw-out roller located at a sheet draw-out position above the tray for feeding the sheets from the tray, and a separation roller pair for driving only the uppermost one of the sheets drawn out by the draw-out roller toward the sheet feed section by separating it from the others. The separation roller pair comprises an upper roller rotatable in the same direction as an intended direction sheet feed and a lower roller rotatable in the opposite direction to the same. When a plurality of paper sheets are fed together by the draw-out roller, the upper roller drives the uppermost sheet toward the sheet feed section while, at the same time, the lower roller returns the other sheets toward the tray.
The prior art sheet feeder described above has the following shortcoming. When the returning force exerted by the lower roller on the sheets becomes greater than the frictional force acting between those sheets, the trailing edges, or leading edges as viewed in the returning direction, of the sheets fail to be aligned with the trailing edges of other sheets which have been left on the tray. As such sheets returned to the tray are fed by the draw-out roller again, the amount of feed by the draw-out roller or the amount of transport by the separation roller pair is varied to in turn disturb the registration timing. This results in incomplete sheet feed and therefore sheet jams.
While various implementations for eliminating incomplete sheet feed as stated above have been proposed, none of them is operable in a satisfactory manner.